Toll-like receptors (TLRs) regulate activation of the innate immune response and influence the development of adaptive immunity by initiating signal transduction cascades in response to bacterial, viral, parasitic, and in some cases, host-derived ligands (Lancaster et al., J. Physiol. 563:945-955, 2005). The plasma membrane localized TLRs, TLR1, TLR2, TLR4 and TLR6 recognize ligands including protein or lipid components of bacteria and fungi. The predominantly intracellular TLRs, TLR3, TLR7 and TLR9 respond to dsRNA, ssRNA and unmethylated CpG DNA, respectively. Dysregulation of TLR signaling is believed to cause a multitude of problems, and therapeutic strategies are in development towards this axis (Hoffman et al., Nat. Rev. Drug Discov. 4:879-880, 2005; Rezaei, Int. Immunopharmacol. 6:863-869, 2006; Wickelgren, Science 312:184-187, 2006). For example, antagonists of TLR4 and TLRs 7 and 9 are in clinical development for severe sepsis and lupus, respectively (Kanzler et al., Nat. Med. 13:552-559, 2007).
TLR3 signaling is activated by dsRNA, mRNA or RNA released from necrotic cells during inflammation or virus infection. TLR3 activation induces secretion of interferons and pro-inflammatory cytokines and triggers immune cell activation and recruitment that are protective during certain microbial infections. For example, a dominant-negative TLR3 allele has been associated with increased susceptibility to Herpes Simplex encephalitis upon primary infection with HSV-1 in childhood (Zheng et al., Science 317:1522-1527, 2007). In mice, TLR3 deficiency is associated with decreased survival upon coxsackie virus challenge (Richer et al., PLoS One 4:e4127, 2009). However, uncontrolled or dysregulated TLR3 signaling has been shown to contribute to morbidity and mortality in certain viral infection models including West Nile, phlebovirus, vaccinia, and influenza A (Wang et al., Nat. Med. 10:1366-1373, 2004; Gowen et al., J. Immunol. 177:6301-6307, 2006; Hutchens et al., J. Immunol. 180:483-491, 2008; Le Goffic et al., PloS Pathog. 2:E53, 2006).
The crystal structures of the human and murine TLR3 extracellular domains have been determined ((Bell et al., Proc. Natl. Acad. Sci. (USA), 102:10976-80, 2005; Choe, et al., Science 309:581-585, 2005; Liu et al., Science, 320:379-381, 2008). TLR3 adopts the overall shape of a solenoid horseshoe decorated by glycans and has 23 tandem units of leucine-rich repeat (LRR) motifs. The dsRNA binding sites have been mapped to two distinct regions (Liu et al., Science, 320:379-81, 2008). The signaling assembly has been proposed to consist of 1 dsRNA and two TLR3 extracellular domains (Leonard et al., Proc. Natl. Acad. Sci. (USA) 105: 258-263, 2008).
TLR3 has been shown to drive pathogenic mechanisms in a spectrum of inflammatory, immune-mediated and autoimmune diseases including, for example, septic shock (Cavassani et al., J. Exp. Med. 205:2609-2621, 2008), acute lung injury (Murray et al., Am. J. Respir. Crit. Care Med. 178:1227-1237, 2008), rheumatoid arthritis (Kim et al., Immunol. Lett. 124:9-17, 2009; Brentano et al., Arth. Rheum. 52:2656-2665, 2005), asthma (Sugiura et al., Am. J. Resp. Cell Mol. Biol. 40:654-662, 2009; Morishima et al., Int. Arch. Allergy Immunol. 145:163-174, 2008; Stowell et al., Respir. Res. 10:43, 2009), inflammatory bowel disease such as Crohn's disease and ulcerative colitis (Zhou et al., J. Immunol. 178:4548-4556, 2007; Zhou et al., Proc. Natl. Acad. Sci. (USA) 104:7512-7515, 2007), autoimmune liver disease (Lang et al., J. Clin. Invest. 116:2456-2463, 2006) and type I diabetes (Dogusan et al. Diabetes 57:1236-1245, 2008; Lien and Zipris, Curr. Mol. Med. 9:52-68, 2009). Furthermore, organ-specific increases in TLR3 expression have been shown to correlate with a number of pathological conditions driven by dysregulated local inflammatory responses such as in liver tissue in primary biliary cirrhosis (Takii et al., Lab Invest. 85:908-920, 2005), rheumatoid arthritis joints (Ospelt et al., Arthritis Rheum. 58:3684-3692, 2008), and nasal mucosa of allergic rhinitis patients (Fransson et al., Respir. Res. 6:100, 2005).
In necrotic conditions, the release of intracellular content including endogenous mRNA triggers secretion of cytokines, chemokines and other factors that induce local inflammation, facilitate clearance of dead cell remnants and repair the damage. Necrosis often perpetuates inflammatory processes, contributing to chronic or exaggerated inflammation (Bergsbaken et al., Nature Reviews 7:99-109, 2009). Activation of TLR3 at the site of necrosis may contribute to these aberrant inflammatory processes and generate a further pro-inflammatory positive feedback loop via the released TLR3 ligands. Thus, TLR3 antagonism may be beneficial in a variety of disorders involving chronic or exaggerated inflammation and/or necrosis.
Down-modulation of TLR3 activation may also represent a novel treatment strategy for oncologic indications including renal cell carcinomas and head and neck squamous cell carcinomas (Morikawa et al., Clin. Cancer Res. 13:5703-5709, 2007; Pries et al., Int. J. Mol. Med. 21:209-215, 2008). Furthermore, the TLR3L423F allele encoding a protein with reduced activity has been associated with protection against advanced “dry” age-related macular degeneration (Yang et al., N. Engl. J. Med. 359:1456-1463, 2008), indicating that TLR3 antagonists may be beneficial in this disease.
Pathologies associated with inflammatory conditions and others, such as those associated with infections, have significant health and economic impacts. Yet, despite advances in many areas of medicine, comparatively few treatment options and therapies are available for many of these conditions.
Thus, a need exists to suppress TLR3 activity to treat TLR3-associated conditions.